the angle interior formula is
each interior angle=

where n=number of sides
subsitute156= [tex] \frac{(n-2)180}{n} [\tex]
multiply both sides by n
156n=(n-2)180
divide both sides by 180
13/15 times n=n-2
multiply both sides by 15 to clear fraction
13n=15n-30
subtract 13n from both sides
0=2n-30
add 30 to both sides
30=2n
divide 2
15=n
it has 15 sides